1. Field of the Invention
The invention concerns a device to control an electrical supply of a vehicle headlamp. The invention also concerns a motor vehicle headlamp as well as a control procedure for the supply of such a headlamp.
2. Description of the Related Art
The invention is applied in the automobile field and, in particular, in the field of lighting for motor vehicles for the electricity supply to incandescent bulbs or electro-luminescent diodes in vehicle headlamps.
In the field of automobile lighting, there are different types of lighting function, which essentially include the following:                sidelights of low intensity and range. This function is mainly intended to signal the presence of the vehicle in terms of average brightness and visibility;        low beam, of stronger intensity with a range on the road generally approaching 70 meters. This function is essentially used at night. It features a luminous division presenting, in at least one zone, a strong luminous gradient so as not to dazzle drivers in front or oncoming;        high beam, with a long range typically approaching 200 meters. This function is used to light up the road from afar and allow the driver to apprehend the trajectory of the vehicle; and        fog lamps.        
At present, there are light projection devices, or headlamps, which provide low beam and high beam functions. These headlamps are generally referred to as dual function headlamps.
Such a dual function headlamp comprises a light source emitting a flow of light which is identical regardless of the mode of functioning (high or low beam). It generally comprises a movable cover suitable to ensure a cut-off of the beam of light. This cover is, for example, a metal shutter which may be in a first position or a second position. In the first position, the cover does not conceal the beam of light produced by the light source, or bulb, of the headlamp. In the second position, the cover partially conceals the beam of light produced by the headlamp bulb. In the second position, it is said that the cover specifically cuts off the beam of light, and this specific cut-off corresponds to the cut-off of the beam of light necessary to obtain the low beam function. Thus, lighting in low beam or high beam mode is differentiated by the presence or otherwise of the cut-off.
In current dual function headlamps, the light source may be a halogen bulb. This halogen bulb is generally supplied by a fixed voltage, which is generally the rated voltage of the bulb specified by bulb manufacturers for each type of bulb. This rated voltage is quite often the feed voltage for which the functioning of the bulb fulfils statutory requirements.
The bulb manufacturers, in addition to indicating a rated voltage, indicate an average life duration for the bulbs. Typically, the life duration of a bulb of type H7 is about 900 hours at 13.2V.
However, it transpires in practice that the average life duration of the bulbs is often significantly below that indicated by the manufacturers due to the voltage of the vehicle's electrics, which often have a rated voltage higher than that of the bulb. At the end of its life duration, a new bulb must be purchased and re-installed in the headlamp. This operation is increasingly complicated for the user to carry out, and may entail the immobilization of the vehicle and an extra cost.
There is therefore a need to increase the real life duration of the bulbs while conserving satisfactory lighting performances.
Furthermore, there is a need to reduce the energy consumed by a headlamp for lighting the vehicle.